1. Field of the Invention
The present invention generally relates to a catalyst degradation detecting apparatus. More particularly, the present invention relates to a catalyst degradation detecting apparatus in which the concentration of a specific ingredient in exhaust gas is detected by air-fuel ratio sensors provided on an upstream side and downstream side of a catalytic converter which purifies the exhaust gas within an automotive or other type of exhaust system and which generates a signal based on the condition of the degradation of the catalyst. The invention also relates to an air-fuel ratio sensor suitable for use in the above catalyst degradation detecting apparatus.
2. Description of the Related Art
As methods of detecting the degradation of the purifying performance of a catalyst by using air-fuel ratio sensors on the upstream side and downstream side of the catalyst, (1) a method using the ratio of a frequency of outputs from two air-fuel ratio sensors, and (2) a method using an area difference or an area ratio between two air-fuel ratio sensors have conventionally been-available (as disclosed in the Japanese Unexamined Patent Publications Nos. Hei 4-40355, Hei 61-286550). However, since as any of these methods provides ratios or differences of frequencies or areas of the two air-fuel ratio sensors, threshold values are required as reference values against output waveforms of two air-fuel ratio sensors, and depending on the setting method or setting condition for these threshold values, the information obtained from the air-fuel ratio sensors may vary.
In other words, information due to variable factors other than information relating to the catalyst affects the information relating to the variation (degradation) of the purification ratio of the catalyst as described above, and for this reason, the detecting performance is very low.
In addition to this problem, since the air-fuel ratio sensors differ in individual performance characteristics, an erroneous judgment, a suspended judgment or other abnormality may be caused, and the catalyst degradation can not be consistently and reliably detected.
Since differences in responsiveness characteristics of the upstream side and downstream side air-fuel ratio sensors exists, the sensors usually have been selected from the same product lot distribution or the same standard range which are both .+-.10% of the average sensor responsiveness characteristics.